What is the communicator in the Philips Hue app?

The communicator is the device in the Hue system that is designated to control all other lights in the entertainment setup. It is either a Hue light or the Hue bridge basically which can acts as a broadcaster of the entertainment commands.

Selecting the communicator manually will only show the lights which support richer color version bulbs (E26/E27/E12/E14/GU10 and BR30), Hue Go and Hue Lightstrip Plus. These lights have hardware optimized to run entertainment smoothly.